In a gas turbine engine, ambient air is drawn into a compressor section. Alternate rows of stationary and rotating aerofoil blades are arranged around a common axis; together these accelerate and compress the incoming air. A rotating shaft drives the rotating blades. Compressed air is delivered to a combustor section where it is mixed with fuel and ignited. Ignition causes rapid expansion of the fuel/air mix which is directed in part to propel a body carrying the engine and in another part to drive rotation of a series of turbines arranged downstream of the combustor. The turbines share rotor shafts in common with the rotating blades of the compressor and work, through the shaft, to drive rotation of the compressor blades.
It is well known that the operating efficiency of a gas turbine engine is improved by increasing the operating temperature. The ability to optimise efficiency through increased temperatures is restricted by changes in behaviour of materials used in the engine components at elevated temperatures which, amongst other things, can impact upon the mechanical strength of the blades and a rotor disc which carries the blades. This problem is addressed by providing a flow of coolant through and/or over the turbine discs and blades.
It is known to take off a portion of the air output from the compressor (which is not subjected to ignition in the combustor and so is relatively cooler) and feed this to surfaces in the turbine section which are likely to suffer damage from excessive heat.
Typically the cooling air is delivered adjacent the rim of the turbine disc and directed to a port which enters the turbine blade body and is distributed through the blade, typically by means of a labyrinth of channels extending through the blade body.
Turbine blades are known to be manufactured by casting methods. A mould defines an external geometry of the turbine and a core is inserted into the mould to define the internal geometry, molten material (typically a ferrous or non-ferrous alloy) is then cast between the mould and the core and the core subsequently is removed, for example by leaching.
To cool the trailing edge of an aerofoil, holes are positioned as far downstream (with respect to the direction of working fluid flow) as possible. This also reduces the required flow as the pressure margin between the internal and external is lowest at the trailing edge, this improves cooling flow efficiency. Desirably, holes would be located along the apogee of the trailing edge; however, this would require an increased thickness at the apogee which could result in deterioration in aerodynamic efficiency. Since aerofoils for use in gas turbine engines have typically been manufactured by investment casting, it has been difficult to control the consistency of thickness along the apogee of the trailing edge.
It is known to use a system known as Pressure Side Ejection (PSE) to cool the trailing edge of a turbine blade. In such systems, holes are drilled into a cast aerofoil near the apogee but on the pressure surface side. Electrical Discharge Machining (EDM) is known to be used to drill the holes. The surface is subsequently finished to provide a smooth surface and maintain efficient flow over the aerofoil.
The described PSE system has been improved upon with the use of adaptive machining. Using adaptive machining process, apogee thickness, which by casting capability alone is limited to the order of 0.6 mm nominal can be reduced. The thickness of the apogee need only then be limited to reduce handling damage with thickness of around 0.2-0.5 mm nominal achieveable. Adaptive machining of the trailing edge is conventionally undertaken prior to drilling of cooling holes. However this does not improve the EDM capability as the drilled holes must be positioned away from the apogee to reduce variation due to the uneven surface. Also the shallow entry angle of an EDM tool increases scarring of the drilled surface of the aerofoil.
U.S. patent publication no. US8770920B2 discloses an arrangement wherein a step is cut into the apogee of a trailing edge. This allows for holes to be drilled through a thicker section of the aerofoil, upstream of the apogee. The step also results in a reduced thickness at the apogee. A scalloped surface extends forward of the step to the apogee. The presence of this step on the pressure or surface side of the aerofoil can have a detrimental effect on flow as it passes the trailing edge.